Grinding mills find application in ore processing and agglomeration processes, such as, for example, sintering, briquetting, and pelletizing. The mills may be bar mills, ball mills, semi-autogenous, or autogenous mills, that is a mill wherein the raw or feed material is the grinding media. The grinding apparatus may consist of a drum, an elongated cylinder, a barrel or other enclosure for retaining the raw material during the grinding cycle, as well as input devices to feed the raw material and output devices to separate, handle or otherwise distribute the ground material. A specific example of such a mill is an autogenous mill, which has a cylindrical drum and which is rotatable so as to tumble the raw material on itself for grinding the raw material to a finer mesh size. A feed chute allows for continual input to the drum through means of a port at one end of the drum, and a trommel screen arrangement at the opposite end of the drum is provided to screen the fine material and to recycle the oversize materials for further reduction in size.
The transfer of raw material to the trommel screen may be accommodated by means of an overflow or weir-like operation by as a result of the continual input of raw material thus forcing other material out the second end of the drum. However, this type of ground material transfer is not very efficient as it is not size selective and does not accommodate for a predetermined residence time within the grinding drum material above a predetermined size. Therefore, discharge apparatus are utilized to selectively cull material from the grinding stock and transfer it to the trommel screen for segregation of a finished size material and for recharging screened oversize material to the drum for further grinding. These grinding, charging and screening functions are utilized within ore processing facilities such as, for example, in the preparation of taconite ores for grinding, beneficiation and agglomeration into pellets for smelting, foundry and blast furnace operations.
The apparatus for transferring the ground material to the trommel screen is a pulp discharge vein or chute, which includes a coarse sieve-like grate operatively associated with the drum shell so as to capture some of the ground material smaller than the grate hole size for transfer through the chute to the trommel screen during the rotation of the drum, transfer chutes and trommel screen. The transfer chutes generally extend from the discharge end opening to the drum inside diameter and appear as rays or radii generally extending from the center of a circle to the circumference thereof. The chutes or veins are open at the discharge port to the trommel screen or other means, and as the drum rotates through its cycle the material gathered at the grate of each chute is gravity-fed to the chute opening for discharge to the screen. In an exemplary apparatus, the chute open ends are arranged as an annulus so as to provide a return tube port at the center of the discharge annulus for a tube returning the coarse material to the drum for further grinding.
The ore bed or raw material within the drum is generally below the level of the discharge port and had previously been at a depth that was positioned along the discharge chute length, which discharge tubes were fabricated from a wearable metal so as to avoid the abrasion from the ores. However, in an effort to increase production and utilization of plant and equipment the users have increased the bed depth within the drum and established the bed/air interface at the rear surface of the discharge cone of the chute. The discharge cone length was previously a one-piece design with a polyurethane coating which survived processing of 8.0 million tons, as the bed level did not impinge upon its surface. However, the increased height of the bed provided abrasive interference at the mill or outer side of the discharge cone, which caused premature cone erosion at approximately 4.0 million tons of processed ore. Makeshift metal wear plates are presently bolted onto the cones utilizing the cone mounting bolts securing the cone to the drum end plate, which again raises the cone wear life to 8.0 million tons. The replacement of these metal wear plates is, however, time-consuming since the bolts are commonly shared with the discharge cone.
Another attempt to extend the cone life utilized thicker wear plates embedded within the polyurethane casting. This approach fell short of the 8.0 million ton target and metal wear plates were again required to be bolted to the cone.
This extensive wear plate replacement procedure and discharge cone replacement at 8.0 million tons has prompted research into means for extending the service life of the cone wearing components and making the high wear components easier to replace so as to reduce mill down time.